Some gestures have been removed in Firefox 4+ versions.

You can restore the zoom feature by changing the values of the related prefs on the about:config page.

• browser.gesture.pinch.in -> cmd_fullZoomReduce
• browser.gesture.pinch.in.shift -> cmd_fullZoomReset
• browser.gesture.pinch.out -> cmd_fullZoomEnlarge
• browser.gesture.pinch.out.shift -> cmd_fullZoomReset
• browser.gesture.pinch.latched -> false

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• Use the Filter bar at to top of the about:config page to locate a preference more easily.
• Preferences that have been modified show as bold (user set).
• Preferences can be reset to the default via the right-click context menu if they are user set
• Preferences can be changed via the right-click context menu: Modify (String or Integer) or Toggle (Boolean)

Thank you, that does the trick; the question arising, why did this have to be changed in the first place?

People used to change things by accident and those gestures were removed to prevent that.

• Bug 613909 – Disable pinch gestures by default & revert three finger vertical swipe gestures